I admit I'm a bit out of my technological depth here so this may be an obvious question but does a master node maintain anonymity?
A follow-up question if the answer is 'no': Could there be grounds for the prosecution of a master node operator for money laundering?
Behind TOR, yes.
As for your second question, mixing coins is not money laundering as far as I know, it's a privacy tool.
edit:
Found this:
https://en.bitcoin.it/wiki/Anonymity\Legality
Obviously, using Bitcoin anonymity techniques for the purposes of money laundering is illegal, but participating or running schemes which others use for money laundering may also be illegal or at least leave the participant vulnerable to accusations of aiding criminals and terrorists. Bitcoin anonymity techniques involving bitcoins worth large amounts of money (over some value between FJ$1,100 and US$58,000, depending on your jurisdiction) is illegal in most jurisdictions, being in violation of anti-structuring laws.
double edit:
Going by this:
http://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1027&context=law_facpub1) You would have to provide the service knowing that the other party was structuring/laundering (you wouldn't).
2) They would have to be able to prove through tracing many DarkSends that those parties even owned the coins. Next to impossible unless sybil attack somehow happens on a wide scale (highly unlikely from what I've read by Evan).
3) Most of those sections have to do with a user declaring their own money or a receiving business declaring the money, both of which would not apply to a master node.